As is well known, an aluminum alloy panel also is used instead of a steel (steel sheet) panel for an outer panel (outer sheet) and an inner panel (inner sheet) and the like of a panel structure such as a hood (bonnet), fender, door, luggage (trunk lid), roof, and the like, of an automobile. For these aluminum alloy panels, similar to the case of steel sheet, a panel formed product (press formed product, hereafter referred simply to also as panel), which is a press molded product of an aluminum alloy sheet (blank, a material sheet for panel forming) by a mold, is used.
Among them, the outer panel such as a hood, luggage and the like has become difficult particularly in press forming because of diversification of automobile (panel) design in recent years also. Its reason originates in inferior forming performance of an aluminum alloy sheet compared to a steel sheet, as well as in the shape itself of these outer panels.
One example of a luggage outer panel is exhibited in the perspective view drawing of a rear part of an automobile body in FIG. 11. As exhibited in FIG. 11, a luggage outer panel 1 comprises a high (large) ridge line part (ridge part) 2 extending in the vehicle body width direction, and two or more long (large) curved face parts (or projected peripheral edge parts, outline parts) 3, 4 surrounding the ridge line part 2, in the center part (top part) of the panel. And this ridge line part 2 often has a character line of a semicircular (curved) shape in a plan view as exhibited in FIG. 11 for the design of the vehicle body. Further, 9 in FIG. 11 is a license seat (recessed part) to which a number plate is mounted.
When such a luggage outer panel 1 having a semicircular large ridge line part is manufactured by stretch forming by a press device from an aluminum alloy sheet which is a forming material sheet, a wrinkle particularly is liable to occur on the formed face surface of the formed product panel. In other words, as a press formed product panel of FIG. 10 described below, a wrinkle is liable to occur in a portion exhibited as X extending from curved faces 3, 4 to a ridge line part 2.
Occurrence of such a wrinkle is a problem peculiar to the case that the product with such shape of the luggage outer panel 1 having the semicircular large ridge line part 2 is stretch formed with an aluminum alloy sheet. This is because the ridge line part 2 pulls the material sheet unevenly, material excess for material flow-in is liable to occur, the material sheet part of an end part does not contact a punch until the vicinity of the bottom dead point (hereafter referred to also as non-contact outer peripheral part), and deforms freely. Such problem of occurrence of a wrinkle is a problem peculiar to the formed product panel having the non-contact outer peripheral part. In other words, in an ordinary sheet-like panel formed product without such a non-contact outer peripheral part or not in the shape with the ridge line part 2 and the curved face parts 3, 4, the problem of occurrence of such a wrinkle is hard to be caused.
Ordinary method in the case such luggage outer panel 1 is stretch formed by a press device is explained by FIGS. 12(a), (b) exhibiting the condition of the sheet and the press device during pressing. FIG. 12(a) is a perspective view drawing showing the deform condition of the aluminum alloy sheet in the course of pressing, and FIG. 12(b) is a perspective view drawing when the press device and the aluminum alloy sheet are cut and viewed at section B of FIG. 12(a). Ordinarily, an aluminum alloy sheet 30 is formed, with a peripheral edge part 31 of the aluminum alloy sheet 30 being held by a dice 22 and a blank holder (a wrinkle press member) 20, and with the dice 22 being moved (lowered) relatively toward the punch 11 fixed on a bolster (base) 21.
Then, in the aluminum alloy sheet during forming, parts of sheets 33, 34 between a top part 32 corresponding to the ridge line part 2 of the luggage outer panel and the peripheral part 31 held by the blank holder 20 correspond to the curved face parts 3, 4 of the luggage outer panel.
In the panel in the course of forming exhibited in FIG. 12(a), section A in the center part of the product is in a shape wherein its peak is higher than that of section B which is in a position shifted from the center of the product because the ridge line of the panel is in a nearly arc shape. In other words, the line length 1A (1A′) of section A is longer than the line length 1B (1B′) of section B, and line length difference occurs between both sections A, B. When line length difference occurs between neighboring 2 sections like this, uneven tensile force is liable to act as described previously and material excess is liable to occur, therefore the non-contact outer peripheral part occurs wherein the material sheet part of the end part does not contact with the punch until the vicinity of the bottom dead point. Because this non-contact outer peripheral part freely deforms, a wrinkle becomes liable to occur in X and the like exhibited in the drawing as described above.
As a countermeasure to this, as the section of the press device is exhibited partly in FIG. 13, the method wherein excess material forming parts 19, 26 are provided in inner side of the blank holder 20 and tensile force accompanying excess material forming is applied to the sheet 30 is common. In other words, the method is commonly used wherein, by providing this excess material in the periphery part of the sheet 30, the sheet 30 contacts a mold which shapes the excess material part from early stage of press forming and tensile force is applied during shaping of excess material, thereby occurrence of a wrinkle described above is prevented.
Also, apart from this, a method is proposed, wherein, when an aluminum alloy hood outer panel having a large folding part in its end part is press formed, the forming face center part of a sheet where occurrence of recessed parts becoming the cause of a wrinkle is foreseen is formed beforehand prior to start of wrinkle pressing or upon start of wrinkle pressing (refer to the patent document 1, for example). In this method, the height of the panel (product) forming face of the punch is raised by a predetermined size from the top face of the wrinkle press member, and the forming face center part of the sheet is formed beforehand.
Further, a method is also proposed wherein an aluminum alloy sheet is press formed molded to an automobile hood outer panel which has a shape having a large folding section and a border line connecting this folding section and a panel body part being curved toward the front of the panel body (refer to the patent document 2, for example). In other words, this is a method wherein, so that a part corresponding to the wrinkle press face during press forming out of the parts of sheet end part corresponding to the folding section becomes a shape adapting to the wrinkle press member face shape during press forming, the aluminum alloy sheet end part is bent beforehand and a bending part is formed, thereafter press molding is carried out with wrinkle pressing being performed for the portion including a part corresponding to the wrinkle press face of the folding section. In addition, it is also proposed that the prior bending work of the aluminum alloy sheet end part is performed by a divided punch which moves independently from the part of the punch for forming of the panel body.    Patent document 1: Japanese Unexamined Patent Application Publication No. 2001-58218 Bulletin (whole text)    Patent document 2: Japanese Unexamined Patent Application Publication No. 2004-188445 Bulletin (whole text)